Sametz: CHEM 321 Fall 2011 Organic Chemistry Final

Transcription

1 Name: (Print your name clearly!) AM Sametz: CHEM 321 Fall 2011 Organic Chemistry Final All answers should be written CLEARLY in the space provided. (If it s not clear, it s wrong). You may raise your hand to ask a question if you are unsure what a question is asking of you.

2 1. (6 points) Rank the following compounds on order of decreasing acidity (1 = most acidic, 7 = least). 2. (5 points) Draw all possible isomers of C 4 H 6. Hint: there are 9. Points will be subtracted for redundant or incorrect structures, so be concise. 3. (8 points) For each of the following pairs of structures, indicate if they represent a pair of diastereomers, enantiomers, constitutional isomers, or identical compounds. a) b) c) d)

3 4. (8 points) Chloride A reacts with sodium methoxide to give alkene B as a single enantiomer. However, chloride C (a diastereomer of A) reacts with sodium methoxide to give B as a racemic mixture. Explain why.

4 5. The following two reaction sequences both convert cis-2-butene to 2,3-butanediol. However, they give different diastereomers. a) (4 points) Both reactions provide product(s) with no net optical activity.. Which reaction provides a single meso-isomer as a product, and which provides a (dl)- (i.e. racemic) mixture of two enantiomers? b) (8 points) Explain the differences in stereochemical outcome. Hint: a picture is worth a thousand words.

5 6. Consider a carbene such as :CH 2 and answer the following questions: a) (2 points) By VSEPR rules, what is its electronic geometry (i.e. the relative orientation of the two bonding orbitals and single lone pair)? b) (2 points) What is the hybridization of the carbon atom? c) (4 points) Explain why carbenes can act both as a nucleophile and an electrophile.

6 7. The following reaction gives a mixture of two alkene products. One is the kinetically favored product, and the other is the thermodynamically favored product. a) (2 points) Which do you expect to be the thermodynamic product? Explain why. b) (2 points) Which do you expect to be the kinetic product? Explain why this alkene product is formed faster than the thermodynamic product. c) (4 points) Complete the following energy vs. reaction progress diagram. Place the correct products at the correct relative energy level. Use a solid line to represent the energy along the pathway to the thermodynamic product. Use a dashed line to represent the energy along the pathway to the kinetic product. Using the diagram, point out which energy difference is responsible for thermodynamic control and which energy difference is responsible for kinetic control.

9 10. Reaction mechanisms: a) (6 points) Show a reasonable reaction mechanism for ONE of the following two reactions. If you show work on more than one, CLEARLY indicate which one you wish to count for credit; otherwise, only the first will be graded. i) ii)

10 b) (8 points) Show a reasonable reaction mechanism for ONE of the following two reactions. If you show work on more than one, CLEARLY indicate which one you wish to count for credit; otherwise, only the first will be graded. i) ii) (must account for formation of both products)

11 11. (12 points) Multistep Synthesis: Choose TWO of the following two synthesis problems below. Show how you can make the final product from the given starting material. The syntheses will require more than one chemical step. Retrosynthetic analysis is worth part credit, but for full credit write out the forward sequence of reactions with specific reagents. Mechanisms are not required and are not graded. If you show work on more than two problems, CLEARLY indicate which you want graded; otherwise the first two on the page will count for credit. a) b) c) d)

12 (Extra work space for Question 11)

13 EXTRA CREDIT: 20 points. The following questions involve the chemistry of certain compounds associated with the food and lore of the holiday season. 12. (4 points) The principal component of cinnamon s flavor is cinnamaldehyde. It gradually oxidizes in air to cinnamic acid: A friend of mine was once purifying an old bottle of cinnamaldehyde by extracting the impure aldehyde with aqueous sodium bicarbonate. His 4-liter separatory funnel built up pressure, and he was soaked with the contents. The burn of the cinnamon was agonizing, and it wouldn t wash off with water. His coworker had to give him ethanol to swab the cinnamaldehyde off his skin. a) What was the idea behind extracting the old, impure cinnamaldehyde with bicarbonate? Which component was in the organic layer, and which was in the aqueous layer? b) Why is pressure buildup such a common danger with extractions using carbonate or bicarbonate? What do you do in the lab to avoid such a pressure buildup? c) Why was ethanol more effective at washing cinnamaldehyde off of his skin than water?

14 13. (4 points) Eugenol is the main component of clove oil. Vanillin is the major component of vanilla, and is commonly used as artificial vanilla flavor. Vanillin has been synthesized from eugenol. Design your own synthesis of vanillin from eugenol.

15 14. (2 points) Gingerol is a major flavor component of ginger root. Dried, powdered ginger is more pungent than fresh ginger. This is partially attributed to the dehydration of gingerol to shogaol: Give a set of reaction conditions that might convert gingerol to shogaol. 15. Menthol is a component of peppermint oil. It is most commonly the (-) enantiomer shown below. a) (3 points) Assign each stereocenter as R or S-. b) (3 points) Draw the more stable chair conformer of (-)-menthol.

16 c) (4 points) Menthol can be produced from myrcene, which is found in trace amounts in myrrh (one of the three gifts of the Magi in the biblical account of the birth of Jesus). The formula of myrcene is C 10 H 16. It reacts with excess hydrogen gas and a platinum catalyst to provide the hydrocarbon C 10 H 22 : Ozonolysis of myrcene followed by addition of hydrogen peroxide produces the following compounds: From this data, determine the chemical structure of myrcene.

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